Photovoltaic device

ABSTRACT

A photovoltaic device manufacturing method is disclosed. Methods include manufacturing a photovoltaic cell using nanoimprint technology to define individual cell units of the photovoltaic device. The methods can include providing a substrate; forming a first conductive layer over the substrate; forming first grooves in the first conductive layer using a nanoimprint and etching process; forming an absorption layer over the first conductive layer, the absorption layer filling in the first grooves; forming second grooves in the absorption layer using a nanoimprint process; forming a second conductive layer over the absorption layer, the second conductive layer filling in the second grooves; and forming third grooves in the second conductive layer and the absorption layer, thereby defining a photovoltaic cell unit.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of and claims benefit of U.S. patent application Ser. No. 12/823,667 filed Jun. 25, 2010, the entirety of which is hereby incorporated by reference herein.

TECHNICAL FIELD

The present disclosure relates generally to photovoltaic cells, and more particularly, to a photovoltaic cell manufacturing.

BACKGROUND

Photovoltaic cells (also referred to as solar cells) convert light energy into electricity. Photovoltaic cells and manufacturing thereof are continually evolving to provide higher conversion efficiency. For example, thin film photovoltaic cells have been introduced to provide improved conversion efficiency. Conventional methods for manufacturing thin film photovoltaic devices use laser and mechanical scribing to define, interconnect, and isolate the photovoltaic device into individual, connected photovoltaic cells. It has been observed that these methods can result in low throughput and photovoltaic cell surface damage. Accordingly, although existing methods have been generally adequate for their intended purposes, they have not been entirely satisfactory in all respects.

SUMMARY

The present disclosure provides for many different embodiments. According to one of the broader forms of embodiments of the present invention, a method for manufacturing a photovoltaic device includes: providing a substrate; forming a first conductive layer over the substrate; forming first grooves in the first conductive layer using a nanoimprint and etching process; forming an absorption layer over the first conductive layer, the absorption layer filling in the first grooves; forming second grooves in the absorption layer using a nanoimprint process; forming a second conductive layer over the absorption layer, the second conductive layer filling in the second grooves; and forming third grooves in the second conductive layer and the absorption layer, thereby defining a photovoltaic cell unit.

In another one of the broader forms of embodiments of the present invention, a method includes: providing a substrate; forming a first conductive layer over the substrate; performing a first nanoimprint process, thereby forming a first opening within the first conductive layer that exposes the substrate; forming an absorption layer over the first conductive layer, the absorption layer filling the first opening; performing a second nanoimprint process, thereby forming a second opening within the absorption layer that exposes the first conductive layer; forming a second conductive layer over the absorption layer, the second conductive layer partially filling the second opening; and removing portions of the second conductive layer and the absorption layer, thereby forming a third opening that exposes the first conductive layer.

Yet another one of the broader forms of embodiments of the present invention involves a method for manufacturing a thin film solar cell. The method includes: providing a substrate; performing a first nanoimprint process to form a first electrode over the substrate; performing a second nanoimprint process to form an absorption element over the first electrode; and forming a second electrode over the absorption element

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale and are used for illustration purposes only. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIG. 1 is a flow chart of a method for fabricating a photovoltaic device according various embodiments of the present disclosure.

FIGS. 2-14 are various diagrammatic sectional side views of a photovoltaic device during various fabrication stages according to the method of FIG. 1.

DETAILED DESCRIPTION

It is understood that the following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Conventional photovoltaic device manufacturing uses laser or mechanical scribing for defining, interconnecting, and isolating individual cells of a photovoltaic device. For example, patterning the individual cells often requires a three step process: a laser scribing process (for example, to define an electrode, such as the back surface field) and then two mechanical scribing processes (for example, one process to define an interconnect, and one process to define the individual cell units). These processes often result in damage to the photovoltaic cell surface and reduce conversion efficiency and throughput. In contrast, as described in detail below, the disclosed photovoltaic device manufacturing uses nanoimprint technology to define, interconnect and isolate individual cells of the photovoltaic device. Replacing laser and/or mechanical scribing with nanoimprint technology can improve throughput (for example, by eliminating electron-hole pair lateral migration issues) and reduce surface damage to the photovoltaic cells. Further, nanoimprint technology provides high patterning accuracy, which can provide improved alignment within the photovoltaic cells. When compared to conventional manufacturing methods, nanoimprint technology is easily implemented into conventional roll-to-roll manufacturing processes, simplifying manufacturing of photovoltaic devices and reducing manufacturing costs. It is understood that different embodiments may have different advantages, and that no particular advantage is necessarily required of any one embodiment.

FIG. 1 is a flow chart of an embodiment of a method 100 for fabricating a photovoltaic device. As will be discussed further below, the method 100 is utilized to define various features (elements) of a photovoltaic device. For example, the method 100 defines electrode portions, absorption portions, and individual photovoltaic cell units of the photovoltaic device. The method 100 begins at block 102 where a substrate is provided. At blocks 104 and 106, a first conductive layer is formed over the substrate, and a first nanoimprint process is performed to form first grooves in the first conductive layer. At blocks 108 and 110, an absorption layer is formed over the first conductive layer, filling in the first grooves, and a second nanoimprint process is performed to form second grooves in the absorption layer. At block 112, a second conductive layer is formed over the absorption layer, filling in the second grooves. The second conductive layer may only partially fill in the second grooves. At block 114, third grooves are formed in the second conductive layer and absorption layer, thereby defining a photovoltaic cell unit. According to various embodiments, the nanoimprint processes utilize thermal nanoimprinting lithography techniques (including thermoplastic and thermal-curable nanoimprinting), direct imprinting techniques (also referred to as embossing), UV nanoimprinting lithography (UV-NIL) techniques (also referred to as UV-curable nanoimprinting), or combinations thereof. Alternatively, the nanoimprint technology utilizes other nanoimprinting lithography (NIL) techniques known in the art, including future-developed NIL lithography techniques, and combinations thereof. The NIL process is performed in a suitable environment, such as a vacuum environment or an air environment. The NIL process uses various alignment techniques, and the nanoimprint process may utilize an etching process, such as a dry etching process, wet etching process, other suitable etching process, or combinations thereof. Additional steps can be provided before, during, and after the method 100, and some of the steps described can be replaced or eliminated for additional embodiments of the method. The discussion that follows illustrates various embodiments of a photovoltaic device that can be fabricated according to the method 100 of FIG. 1.

FIGS. 2-14 are diagrammatic sectional side views of a photovoltaic device 200 (also referred to as a solar cell), in portion or entirety, at various stages of fabrication according to the method of FIG. 1. The photovoltaic device 200 is a thin film solar cell. FIGS. 2-14 have been simplified for the sake of clarity to better explain the inventive concepts of the present disclosure. Additional features not shown can be added in the photovoltaic device 200, and some of the features described below can be replaced or eliminated for other embodiments of the photovoltaic device 200.

In FIG. 2, a substrate 210 is provided having a conductive layer 220 formed thereover. The substrate 210 is any substrate suitable for thin film solar cells. In the depicted embodiment, the substrate 210 is a glass substrate. The glass substrate may be soda lime silicate glass, aluminosilicate glass, lithium aluminosilicate glass, quartz glass, borosilicate glass, alkali-free glass, other suitable glass, or combinations thereof. Alternatively, the substrate 210 is a metal foil (for example, stainless steel, aluminum, or copper) or flexible transparent polymer (for example, polyimide, polyamide, polyethersulfone, polyetherimide, polyethylene naphthalate, or polyester). The substrate 210 may be a flexible substrate.

The conductive layer 220 is disposed over the substrate 210. In the depicted embodiment, the conductive layer 220 is a molybdenum (Mo) layer. The conductive layer 220 is a contact (electrode) layer of the photovoltaic device 200. The conductive layer 220 is a back contact (electrode) layer, and thus, provides a back surface electric field (BSF) for the photovoltaic device 200. Alternatively, the conductive layer 220 may be a Ti, Ni, Al, Nb, W, Cr, Cu, transparent conducting oxide (TCO) layer, other suitable material layer, or combinations thereof.

Referring to FIGS. 3-7, the disclosed photovoltaic cell process defines and patterns the back contact layer, which in the depicted embodiment is the conductive layer 220, using nanoimprint technology. In FIG. 3, a material layer 230 (also referred to as an intermedium or shielding layer) is formed over the substrate 210 and conductive layer 220 by a spin coating, flat scrubbing, or other suitable process. The material layer 230 is a resist layer. The resist layer is a homopolymer resist, such as PMMA (polymethylmethacrylate) or PS (polystyrene); thermal plastic resist; UV-curable resist; resist including siloxane copolymers, such as PDMS (poly(dimethyl siloxane))-organic block or graft copolymers; thermally curable liquid resist; UV-curable liquid resist (for room temperature nanoimprinting, for example); other suitable resists known in the art; future-developed resist; or combinations thereof. In the depicted embodiment, the material layer 230 is a PMMA layer. The material layer 230 may comprise a multi-layer structure. The material layer 230 has a suitable thickness, for example, less than or equal to about 1 μm.

Referring to FIGS. 4-7, a mold 240 is pressed into the material layer 230 and removed, thereby imprinting the material layer 230 with a predetermined pattern. The mold 240 is made of a suitable material, such as quartz (SiO₂), silicon, SiC, silicon nitride, metal, sapphire, diamond, resin, other suitable mold material known in the art, future-developed mold material, or combinations thereof. In an example, the mold 240 may be quartz having a patterned metal layer, such as chromium (Cr), forming the predetermined pattern. In another example, the mold 240 may be quartz having a patterned MoSi layer forming the predetermined pattern. The mold 240 includes protrusion features 241 and openings 242 (also referred to as trenches or cavities) that form the predetermined pattern. The predetermined pattern is any suitable design, and thus, the protrusion features 241 and openings 242 may have various shapes and designs depending on a particular pattern or feature desired. In the depicted embodiment, the predetermined pattern of the mold 240 defines the BSF pattern, particularly the pattern of the conductive layer 220.

As noted above, the mold 240 is pressed into the material layer 230 (FIGS. 4 and 5) at a suitable temperature and pressure, thereby creating a thickness contrast in the material layer 230 to form a patterned material layer. More specifically, the predetermined pattern of the mold 240 is transferred to the material layer 230 because the material layer 230 underneath the protrusion features 241 is displaced and transported to the trenches or cavities 242 of the mold 240 (FIG. 5). The temperature and pressure is selected based on properties of the mold 240 and material layer 230, and the imprinting is performed in a vacuum or in air. The material layer 230 is cured and set so that the material layer 230 hardens and assumes its displaced shape. This ensures that the material layer 230 will not flow back into the spaces created by the displacement when the mold 240 is removed. For example, where the material layer 230 is a thermal resist, the temperature may be raised higher than its glass transition temperature so that the material layer 230 changes to a liquid state, such that it is displaced and transported into the trenches or cavities 242 of the mold 240. Once the material layer 230 conforms to the pattern of the mold 240, the temperature may be brought below the material layer's glass transition temperature to solidify the material layer 230. In another example, where the material layer 230 is a thermal or UV curable material, the material layer 230 may initially be in a liquid state, such that it conforms to the mold 240 when pressed into the material layer 230, and then, solidifies when cured by a thermal curing, UV curing, or combination thereof. Other curing and setting processes may be used. When the mold 240 is removed, a patterned material layer 230A remains as illustrated in FIG. 6. Openings (trenches or grooves) 242 and 244 expose portions of the conductive layer 220. The patterned material layer 230A shields other portions of the conductive layer 220 from subsequent processing (such as an etching process). A thin residual layer of the material layer 230 may remain over the exposed portions of the conductive layer 220.

Referring to FIG. 7, an etching process 250 is performed on the conductive layer 220. The etching process 250 is a dry etching process. An exemplary dry etching process is a plasma etching process that has etching parameters that can be tuned, such as etchant used, etching pressure, radio-frequency (RF) source power, and RF bias power. For example, the etching parameters may be an etchant including SF₆, CF₄, O₂, or combinations thereof; an etching pressure of about 3 mili-Torr (mT) to about 8 mT; an RF source power of about 120 Watts (W_(s)) to about 400 W_(s); and an RF bias power of about 12 W_(B) to about 30 W_(B). In situations where a residual layer of the material layer 230 remains over the exposed portions of the conductive layer 220, the etching process 250 removes the residual layer, or a dry etching process, such as a reactive ion etching (RIE) process, may be utilized to remove the residual layer prior to performing the etching process 250.

The etching process 250 transfers the pattern (or design) of the patterned material layer 230A to the conductive layer 220, thereby forming patterned conductive layer 220A. More specifically, the etching process 250 extends the openings 242 and 244 in the patterned material layer 230A to the conductive layer 220, thereby forming openings (trenches or grooves) 252 and 254 in the patterned conductive layer 220A. The openings 252 and 254 isolate portions of the conductive layer 220 from one another, and thus, may be referred to as isolation grooves. Thereafter, the remaining portions of the patterned shielding layer 230A are removed by a suitable process, such as a stripping process, leaving electrodes 256 (FIG. 8). For example, the remaining portions of the patterned material layer 230A may be removed by a solution including sulfuric acid (H₂SO₄) and hydrogen peroxide (H₂O₂). Alternatively, other solutions known in the art, including future-developed solutions, are used for removing the patterned material layer 230A. The remaining conductive layer 220 defines the back contact (BSF) for the photovoltaic device 200.

Referring to FIG. 8, an absorption layer 260 is formed over the substrate 210 and conductive layer 220. The absorption layer 260 fills the openings 252 and 254 in the conductive layer 220. The absorption layer 260 may provide electrical insulation between adjacent electrodes 256. The absorption layer 260 may be selected from a group consisting of Group II-VI compounds, Group III-V compounds, Group IV semiconductors, and combinations thereof. In the depicted embodiment, the absorption layer is a copper, indium, gallium, and selenium (CuIn_(1-x)Ga_(x)Se₂) (CIGS) layer. Alternatively, the absorption layer 260 is a silicon (Si) ink layer, such as an amorphous silicon layer. In yet another alternative, the absorption layer 260 includes GaAs, CdTe, Cu₂S, Cu₂O, InP, Zn₃P₂, other suitable materials, or combinations thereof. The CIGS layer is formed by a process known in the art, including future-developed processes. For example, a vacuum-based process may co-evaporate or co-sputter copper, gallium, and indium to form a film over the substrate, and then anneal the film with a selenide vapor to form the final CIGS layer. In another example, copper, gallium, indium, and selenium may be directly co-evaporated onto the substrate when the substrate is heated.

Referring to FIGS. 9-11, nanoimprint technology is also used to define and pattern the absorber layer 260. More particularly, a mold 270 is pressed into the absorption layer 260 and removed, thereby imprinting the absorption layer 260 with a predetermined pattern. The mold 270 is made of a suitable material, such as quartz (SiO₂), silicon, SiC, silicon nitride, metal, sapphire, diamond, resin, other suitable mold material known in the art, future-developed mold material, or combinations thereof. In an example, the mold 270 may be quartz having a patterned metal layer, such as chromium (Cr), forming the predetermined pattern. In another example, the mold 270 may be quartz having a patterned MoSi layer forming the predetermined pattern. The mold 270 includes protrusion features 271 and openings 272 (also referred to as trenches or cavities) that form the predetermined pattern. The predetermined pattern is any suitable design, and thus, the protrusion features 271 and openings 272 may have various shapes and designs depending on a particular pattern or feature desired. In the depicted embodiment, the predetermined pattern of the mold 270 defines individual cell units of the photovoltaic device 200.

The mold 270 is pressed into the absorption layer 260 (FIGS. 9 and 10) at a suitable temperature and pressure, thereby creating a thickness contrast in the absorption layer 260. More specifically, the predetermined pattern of the mold 270 is transferred to the absorption layer 260 because the absorption layer 260 underneath the protrusion features 271 is displaced and transported to the trenches or cavities 272 of the mold 270 (FIG. 10). The temperature and pressure is selected based on properties of the mold 270 and absorption layer 260. The absorption layer 260 is cured and set so that the absorption layer 260 hardens and assumes its displaced shape. This ensures that the absorption layer 260 will not flow back into the spaces created by the displacement when the mold 270 is removed. When the mold 270 is removed, a patterned absorption layer 260A remains that includes openings (grooves or trenches) 274 and 276 as illustrated in FIG. 11. The openings 274 and 276 expose portions of the electrodes 256 and isolate the absorption layer 260 into individual absorption elements 278. The patterned absorption layer 260A shields other portions of the electrodes 256 from subsequent processing. A thin residual layer of the absorption layer 260 may remain over the exposed portions of the electrodes 256, which may be removed by a suitable process. Alternatively, similar to the process used to pattern the conductive layer 220 described above, the absorption layer 260 could be patterned and defined by: forming a shielding (or intermedium) layer, similar to material layer 230, over the absorption layer; patterning the shielding layer with the mold 270, such that openings are formed in shielding layer that expose portions of the absorption layer 260; and etching the exposed portions of the absorption layer 260, thereby transferring the pattern of the shielding layer to the absorption layer 260 to form the patterned absorption layer 260A. In yet another alternative, the absorption layer 260 could be patterned by conventional photolithography patterning processes.

Referring to FIG. 12, a conductive layer 280 is formed over patterned absorption layer 260A/absorption elements 278. The conductive layer 280 partially fills the openings 274 and 276 between absorption elements 278, providing electrical communication between the conductive layer 280 and exposed portions of the electrodes 256. The conductive layer may completely fill the openings 274 and/or 276. The conductive layer 280 is a transparent conductive oxide (TCO), such as ZnO, ITO, SnO₂, TiO₂, In₂O₃, CdO, other suitable TCO material, or combinations thereof. The conductive layer 280 may alternatively or additionally be Mo, Ti, Ni, Al, Nb, W, Cr, Cu, other suitable material, or combinations thereof. The conductive layer 280 is a contact (electrode) layer of the photovoltaic device 200. The conductive layer 280 may be doped. A buffer layer may be formed between the absorption layer 260 and the conductive layer 280. The buffer layer may be CdS, CdSe, ZnS, ZnSe, an oxysulfide, other suitable material, or combinations thereof.

Referring to FIGS. 13 and 14, a process 290 divides the photovoltaic device 200 into individual cell units. The process 290 removes portions of the conductive layer 280 and absorption element 278, thereby forming a patterned conductive layer 280A. The patterned conductive layer 280A includes an opening (trench or groove) 292 that isolates portions of the conductive layer 280 from one another, thereby forming electrodes 294. The opening 292 exposes a portion of the electrode 256, and separates the photovoltaic device 200 into individual cell units 296 and 298. In the depicted embodiment, the process 290 is a laser scribing process. Alternatively, the process 290 is a nanoimprint process, a lithography process, an etching process, a mechanical machining process, other suitable process, or combinations thereof.

As discussed above, the disclosed photovoltaic device manufacturing uses nanoimprint technology to define, interconnect and isolate individual cells of the photovoltaic device. Nanoimprint technology provides high patterning accuracy, which can provide improved alignment within the photovoltaic cells. Further, in contrast to conventional manufacturing methods, nanoimprint technology is easily implemented into conventional roll-to-roll manufacturing processes, simplifying manufacturing of photovoltaic devices and reducing manufacturing costs. It is understood that different embodiments may have different advantages, and that no particular advantage is necessarily required of any one embodiment.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A device, comprising: a substrate; a first conductive layer, the first conductive layer comprising a nanometer scale pattern defining a plurality of bottom electrodes, each electrode having an upper surface; an absorption layer formed over the first conductive layer and between the bottom electrodes, the bottom electrodes being separated from one another by the absorption layer, the absorption layer having a nanometer scale pattern therein defining a plurality of grooves extending fully through the absorption layer to the upper surfaces of the bottom electrodes; a second conductive layer, the second conductive layer disposed over the absorption layer and within the plurality of grooves to make electrical contact within the grooves with the upper surfaces of the bottom electrodes; and openings formed through the second conductive layer and the absorption layer to separate adjacent cell units from one another.
 2. The device of claim 1, wherein the first conductive layer is a molybdenum layer.
 3. The device of claim 1, wherein the absorption layer is a CIGS layer or a Si ink layer.
 4. The device of claim 1, wherein the second conductive layer is a transparent conducting oxide layer.
 5. The device of claim 1, wherein the second conductive layer partially fills the plurality of grooves in the absorption layer.
 6. The device of claim 5, wherein the second conductive layer lines walls of the plurality of grooves in the absorption layer.
 7. The device of claim 1, wherein the openings expose portions of the first conductive layer.
 8. The device of claim 1, wherein each cell unit includes a bottom electrode formed from a part of the first conductive layer, an absorption element formed from a part of the absorption layer, and a second electrode formed from a part of the second conductive layer.
 9. The device of claim 8, wherein the first conductive layer is a molybdenum layer, the absorption layer is a CIGS layer or a Si ink layer, and the second conductive layer is a transparent conducting oxide layer.
 10. The device of claim 9, wherein the second conductive layer partially fills the second grooves in the absorption layer.
 11. The device of claim 10, wherein the second conductive layer lines walls of the second grooves.
 12. The device of claim 9, wherein the openings expose portions of the first conductive layer.
 13. The device of claim 9, wherein the substrate is a glass substrate.
 14. The device of claim 1, wherein the substrate is a glass substrate.
 15. The device of claim 1, wherein the bottom electrodes are spaced from one another and the absorption layer fills the space between the bottom electrodes.
 16. A device, comprising: a substrate; a first conductive layer, the first conductive layer comprising a nanometer scale pattern defining a plurality of bottom electrodes, each bottom electrode having an upper surface; an absorption layer formed over the first conductive layer and between the bottom electrodes, the bottom electrodes being separated from one another by the absorption layer, the absorption layer having a nanometer scale pattern therein defining a plurality of grooves extending fully through the absorption layer to the upper surfaces of the bottom electrodes; a second conductive layer, the second conductive layer disposed over the absorption layer and within the plurality of grooves to make electrical contact within the grooves with the upper surfaces of the bottom electrodes, wherein the second conductive layer only partially fills the plurality of grooves in the absorption layer; and openings formed through the second conductive layer and the absorption layer exposing portions of the first conductive layer, the openings separating adjacent cell units from one another, wherein each cell unit includes a bottom electrode formed from a part of the first conductive layer, an absorption element formed from a part of the absorption layer, and a second electrode formed from a part of the second conductive layer.
 17. The device of claim 16, wherein the first conductive layer is a molybdenum layer.
 18. The device of claim 16, wherein the absorption layer is a CIGS layer or a Si ink layer.
 19. The device of claim 16, wherein the second conductive layer is a transparent conducting oxide layer.
 20. The device of claim 16, wherein the second conductive layer lines walls of the plurality of grooves in the absorption layer.
 21. The device of claim 16, wherein the first conductive layer is a molybdenum layer, the absorption layer is a CIGS layer or a Si ink layer, and the second conductive layer is a transparent conducting oxide layer.
 22. The device of claim 16, wherein the bottom electrodes are spaced from one another and the absorption layer fills the space between the bottom electrodes. 